Optimisation of laser driven proton beams by an innovative target scheme

Abstract

Proton beams driven by the target normal sheath acceleration mechanism exhibit large divergence and a broad energy distribution with low particle number at high energy. Such undesirable characteristics of the beam can be controlled and optimised by employing a recently developed helical coil technique, which exploits the transient self-charging of solid targets irradiated by intense laser pulses. Highly chromatic focusing of the broadband proton beams was achieved by employing this technique at the TARANIS laser system, where the selected energy slice was tuned by varying the pitch of the coil. A quasi-collimated, narrow energy band proton beam of ∼ 107 particles at 10 MeV was achieved, through a combination of focussing, energy selection and in-situ post-acceleration. This technique may provide a platform for a generation of compact, all-optical ion accelerators.

Physics of chromatic focusing, post-acceleration and bunching of laser-driven proton beams in helical coil targets

“…This current propagates along the helical coil and produces an electromagnetic pulse inside and outside the helical structure, which acts on the proton beam. Contrary to what has been described in the past publications 30,[35][36][37]44,45 , we have demonstrated that the current pulse does not propagate with a constant phase velocity calculated from the coil geometry (1), but that it is dispersed in velocity due to the frequency dependence of the capacitance and inductance of the helical coil. This dispersion modulates the current intensity during its propagation along the helical coil, negative current pulses appear and lead to a favorable configuration of the longitudinal electric field for the proton bunching.…”

“…This comparison demonstrates that the helical coil has focused a large number of protons which were not emitted originally in its aperture angle. By considering a short current pulse propagating along the helical wire with the velocity v i ≈ c 30,36 , one may expect that the collimated protons are the ones which propagate with the same velocity as the current pulse along the coil axis:…”

“…Escaping hot electrons are charging positively the target, and a short discharge current pulse is generated at the same time with the ion acceleration. This current pulse propagates along the wire turned into a helix 30,[35][36][37][38] and generates an electromagnetic pulse 1 (EMP) 39,40 which focuses, post-accelerates and energy selects the ions. The helix plays a double role of an electromagnetic field guide and delay line for the discharge current pulse.…”

Laser produced electromagnetic pulses: generation, detection and mitigation

“…Proton radiography was successfully applied to measure the electromagnetic wave produced by the interaction of a high-intensity laser pulse with solid targets [48,149] . In more recent experiments, this method was applied for studies of the return current propagating in a wire connecting the target to the ground [49,68,[150][151][152] . Figure 52(a) shows a setup for the investigation of the EMP propagating along a folded meander wire [49,68,150,151] .…”

“…In more recent experiments, this method was applied for studies of the return current propagating in a wire connecting the target to the ground [49,68,[150][151][152] . Figure 52(a) shows a setup for the investigation of the EMP propagating along a folded meander wire [49,68,150,151] . The main laser beam ejects electrons and generates a positive charge on the target, inducing a pulse of return current propagating along the wire to the ground.…”